Heat pump thread

If you divide your previous years gas kWh usage by 2900 you come to the power of the ashp you need to heat your home.

Obviously this is a one calc fits all and assuming no other work is done to insulation level or rad sizes.

I am helping my friend build to passive House standards . It’s a steel frame and sip panel build . It’s not a case of make it airtight , leak down check , and it’s now a passive House. Otherwise I could build one with a marble floor , copper sheet roof , single pane windows and single skin brick wall. Make it airtight with crazy foam and silicon and conjure up air loss figure to passive house standard and it would be like living in a tent.
It’s a underground up total build solution. The level of detail required is pretty high and alot of thought goes into how a steel portal frame can be insulated, the slab , ufh , ashp , window spec ,roof design , log burner with external air supply , kitchen extraction etc .

, log burner with external air supply ,

In a passivhaus? That seems like a stunningly bad idea.

No , it isn’t. You need to buy a specific model that doesn’t draw air from inside the structure.
Otherwise, yes a log burner inside a sealed room using the air from in that room for combustion is really rather dangerous.the mvhr create pressure that affects the burn and could draw CO back into the building.
By buying an external vented model the fire draws cold air in from outside, increased efficiency as you are not using already heated air from inside, then the flue exits normally
So it’s a closed system, apart from a few seconds when the next log is chucked on.

In a passivhaus? That seems like a stunningly bad idea.

I think the point being a Passivhaus is unlikely to need that level of heat, and certainly not the ‘uncontrolled’ and intense heat of a log burner.

Still deciding what to do with our old (1890) 2 bed brick cottage.
Solid brick walls, loft well insulated, new double glazed windows and composite doors. Floors not insulated only about 6” of space under there (checked with remote camera)
Small solar and 5kw battery setup. No existing central heating (or oil or gas) currently running electric radiators using home automation to heat during the lowest Agile rates. TBH we like a cool but obviously not cold house. 5kw log burner in the living room for main comfort.
Do heat pumps stack up on old houses? after all there is only so much can be done to minimise heat loss. (External wall insulation is not an option due to the area and planning)

If the pipe work is done nice we have no issues with it all being exposed and visible as a feature (rather than rip up nice original oak flooring)

hmmmm….

I think the point being a Passivhaus is unlikely to need that level of heat, and certainly not the ‘uncontrolled’ and intense heat of a log burner.

Exactly.

Logburner….

I think the point being a Passivhaus is unlikely to need that level of heat, and certainly not the ‘uncontrolled’ and intense heat of a log burner.

That’s definitely an issue.  The in-laws built a highly insulated/airtight MVHR house with a pellet burner boiler(1) but insisted on an open fire much to their architect’s annoyance. I don’t know how the input ventilation was solved for it (it’s open to the room).  As you’d expect, the uncontrollable heat is a real issue – regularly end up sweating in a t-shirt in winter as David throws another log onto the fire.  Given whats known about the pollution of even the ‘greenest’ wood burners I dont think anyone should be considering them now.  A lifetime of open fires almost certainly a contributory fact in his lung cancer as well.

1) pellet burner looked a sensible tech at the time.  There was a local supplier and UK manufacture and cost was reasonable.  It’s not not local, manufactured in Europe and relatively expensive.  Also needs a bulky storage hopper and much more maintenance. Air or ground source heat pump is the only sensible choice now. 

That’s why there is an 8 X 4 roof lantern that can be opened electrically. If it gets too hot.
The building is in woodland with overhead power so the log burner is back up in case of long term power cuts

If you divide your previous years gas kWh usage by 2900 you come to the power of the ashp you need to heat your home.

Could you please explain that a bit more. Just applied it to mine and ended up with 5.5kw. Seems a bit low.

As an aside, ran the boiler at 35degC output for 6 hours on Saturday. Outside temp was around 10degC max. House struggled to hold temp with each room showing a slow decrease in temp over that time from a starting point comfortable 19 / 18degC.  I thought our house was reasonably well insulated. In fact there’s not much more we can do other than external cladding (which we can’t as in a conservation area.)  Bigger rads needed before we go down the ASHP route! 

That’s why there is an 8 X 4 roof lantern that can be opened electrically. If it gets too hot.

So overheat the house then send it to the sky. Genius.

If it’s a proper passive house they will use the log burner once. Total waste of money

roof lanterns and horizontal glazing are often a total disaster in a passive house unless they are well shaded externally.

What I’ve mainly taken from this whole thread is the high temperatures some people are heating their homes to. 21 and 22C ? Are you running an old people’home ? Or is it a shrine to Bikram Yoga? Are you all walking around stark bollock naked and having to go outside to cool down and leave the doors open ?

What I’ve mainly taken from this whole thread is the high temperatures some people are heating their homes to. 21 and 22C ?

You don’t know what everyone’s actual rooms are like. My thermostat is at 17.5, the heating isn’t running, but it’s 22 or so in here with two adults and the TV on.

Similarly just because the stat is on 21 doesn’t mean it’s actually 21 everywhere.

Could you please explain that a bit more. Just applied it to mine and ended up with 5.5kw. Seems a bit low.

This might help https://protonsforbreakfast.wordpress.com/2022/04/05/what-size-heat-pump-do-i-need-a-rule-of-thumb/

Heat loss – I think @molgrips has mentioned it a few times, and you also raise it. As long as your heating system can at least match your rate of heat loss the house stays at its set temperature. Like you’ve seen, a steeper heat gradient from colder outside temperatures can make it hard to maintain. And I think a few folks, @molgrips included, have described that you need a chunk of energy to raise the house temperature from, for example, your overnight setback temperature.

It’s been said a few times in various ways but the solution to heat loss shouldn’t be to put more energy in, it should be to insulate and stop draughts.

Still looking at an ASHP here as even with higher running costs over our GCH it’s less carbon. Protons for breakfast, in another article, mentions typical household gas heating CO2 output being ~2,500kg/year or similar. Not good.

On lower boiler temperatures, what’s the arithmetic relationship between circulating temperature and efficiency? The difference in steam plume appearance from ours puny dribble of steam from running at 50 Celsius to some neighbours’ at temperatures unknown is noticeable.

On lower boiler temperatures, what’s the arithmetic relationship between circulating temperature and efficiency?

heres a graph from heatgeek, showing return temperature vs efficiency in condensing gas boilers

If you are considering a heat pump, it might be worth factoring in wether or not you have an EV. If you do, you can get some really cheap overnight power on an EV tariff, and then you could keep your house warmer overnight which would reduce consumption during the day, and should save you some cash, I think.

The difference in steam plume appearance from ours puny dribble of steam from running at 50 Celsius to some neighbours’ at temperatures unknown is noticeable.

That’s probably a function of how the boiler has modulated, rather than how much is condensing, I reckon. We see similar steam plumes at different flow temps, but our boiler can’t modulate much.

julians

heres a graph from heatgeek, showing return temperature vs efficiency in condensing gas boilers

Is there a downside to running the boiler cooler if it keeps the house warm enough?

Is there a downside to running the boiler cooler if it keeps the house warm enough?

I’m no expert so take this with a pinch of salt,but no I dont think there is any downside if you have a combi boiler.

If you have a regular boiler with a hot water cylinder then the flow temp of the boiler needs to be sufficient to get the hot water up to your desired temperature and ensure that legionella bacteria cant grow – commonly talked about as being no cooler than 60c which means a flow temp of 67c

I’m no expert so take this with a pinch of salt,but no I dont think there is any downside if you have a combi boiler.

If you have a regular boiler with a hot water cylinder then the flow temp of the boiler needs to be sufficient to get the hot water up to your desired temperature and ensure that legionella bacteria cant grow – commonly talked about as being no cooler than 60c which means a flow temp of 67c

Which is why you should almost certainly fit the manufacturer’s (because most boilers won’t talk to anyone elses) weather compensating controls.  Good boilers will be able to give a high flow temperature only when needed for water and the work in efficient, condensing, mode at a lower flow temp for heating.

legionella bacteria cant grow – commonly talked about as being no cooler than 60c which means a flow temp of 67c

“Usually, there is no growth above 55°C, and a temperature of over 60°C has a bactericidal effect. Thus, the WHO recommends that water be heated and stored at 60°C ”  However, water at the tap should be no higher than 50C due to the risk of scalding so theres a need to balance the two risks.   Ideally you’d have a temp limiter before every hot tap outlet (and I building regs requires them on the bath in new builds) but most homes don’t have that.

Also why 67 – surely that depends on losses between boiler and tank.  If they’re in different parts of the house might be significant but if close together minimal.

It strikes me that boiler controls could be smarter here – you only really need it to reach 60+ for disinfection so it would be enough for that to happen briefly once a week with a lower temperature the rest of the time

Also why 67 – surely that depends on losses between boiler and tank

Id just read that generally speaking your boiler flow temp needs to be 7 deg higher than the target temp for your stored hot water otherwise it could take an age to reach the target temp/may never reach the target temp – I guess its just a rule of thumb as every system is different as you note. I was just playing it safe with my ‘advice’ above.

It strikes me that boiler controls could be smarter here – you only really need it to reach 60+ for disinfection so it would be enough for that to happen briefly once a week with a lower temperature the rest of the time

There are some hot water cylinder thermostats that will set the target temp of the water in the cylinder to 60deg once a week so you can have a lower temperature the rest of the time – no idea how good they are, but I came across them when researching how hot to have my water temp , also I think if you use manufacturer controls some can control the water in the cylinder more smartly than is commonly the case.

note I dont have my hot water cylinder set to 60deg, I set it to about 55deg and am comfortable that this is not a risky approach (in terms of legionella) – but it does seem contra to most advice on the internet

Is there a downside to running the boiler cooler if it keeps the house warm enough?

Only two potential issues:
– Short cycling
– Not being able to heat the hot water enough

There are solutions for both those if you have a sophisticated enough boiler.

Re legionella, you don’t actually need to keep the HW at 65 all the time, especially if you use the whole tank regularly. If you let it sit at say 40C for a long time you will have an issue. Some systems can run a decon cycle once a week, you could also simply fit a timer to your electric immersion and have it come on once a week just after a normal heat programme. There is once again a Heat Geek article about this.

That said, your flow temp might still need to be 70C or so to get the water up to even 50C in a sensible time frame before the boiler starts to short cycle, so you’re never going to get there if your flow temp is 40C unless you have some sort of smartness.

All the glazing is triple glazed , including the roof lantern , and all is fitted with electronic remote control internal blinds . Incl all the horizontal windows.
The log burner will probably be rarely used , and may well be a total waste of money but when your £300k down the road another couple of grand is neither here nor there really .
As previously stated, it’s a back up in case of power cuts , there’s no gas available. It’s a 5kw model in a 250m2 building.

I thought the point of a Passivehaus was that it didn’t need any heating, hence the ‘passive’ part?

@molgrips that is a common misunderstanding of Passivhaus. A hang over in name from a different building concept. @singletrackmind, regarding your blinds to prevent overheating. They need to be external as internal blinds don’t do much as the heat has already made it through the glass. Beware of overheating risk. I am guessing no one has done a phpp model and stress tested the overheating risk.

Based upon the rudimentary calc from singletrackmind, we would need 3.2kw heat pump for a 4 bed detached house. I’d guess we would need potentially slightly higher as most of the gas is used in 5-6 months of the year so not a nice flat graph of usage.

I still need to see how the low flow temp set is affected by proper cold weather and if the house can warm up from the low set back overnight temp. It usually drops to 16.5 (in hallway) on a cold night at the lowest outside temp. 

@singletrackmind the passivhaus trust has a technical guidance on overheating risk and a Phpp plugin you can download from their website.

I’d guess we would need potentially slightly higher as most of the gas is used in 5-6 months of the year so not a nice flat graph of usage.

This is why I asked for the underpinning, as I assumed such a simple rule of thumb accounts for that and wanted to check. I looked into the links provided and whilst there is some additional info, there are caveats (such as adjusting upwards if you live in the northern half of the UK, but YET… no real underpinning. I live in a 5 bed stone built semi, there is no way 5.5kw is enough to keep the house warm in the depths of winter.

Based upon the rudimentary calc from singletrackmind, we would need 3.2kw heat pump for a 4 bed detached house.

I have 9kW of GSHP that keeps my house comfortable from the end of February to mid November. That’s a (very) well insulated 200sqm 4 bed detached, probably a similar foot print and volume of a typical 5 bed in the UK? I’ve got 3 *HUGE* double bedrooms in there, and the living room is almost the same floor space as my last house!).
Only issue is it still has the small radiators from the original “pure” electrical heating system. So I still need to buy bigger rads for a few of the rooms.
Over the winter it struggles (when the daily average drops below about -5) then the house will run at about 17-18 degrees. especially if it’s empty all day.
Coldest temperature i’ve seen in the house was about 13 after we’d been away for a long weekend and the outside temp peaked at about -15 for those days…

TLDR, you probably need about twice the power given in the calc to ensure it works all year round.

Try a higher set back temp. It’s part and parcel of the low 3ff8xient approach.

Has anyone any experience of the Samsung Gen 6 ashp’s?

Getting quotes through to retro fit our oil fired mid 00’s stone build, well insulated detached. They’re speccing a 12kwh Samsung gen 6. Yet to get to the detail on rads etc. 

Looking for advice on questions to ask and things to check for a possible ashp and solar install.

We’ve just moved to a 50s semi with solid brick walls, and doing a two storey side extension and single storey rear extension, effectively covering over half the existing wall area with modern building control level insulated construction. Roof insulation is currently good and easy to top up if needed.

The extension will have under floor heating. I’ve got a quote for a 12kw ashp with sunamp thermal battery and 5kw of solar. The calcs in the quote look good and as this is replacing existing gas boiler I can get the govt grant.

What sort of pitfalls do I need to check for? anything to discuss with installer?

Cheers!

An interesting post by Protons for Breakfast for the end of year

2024: Reasons to be cheerful

Me, though I live in a fair-sized Victorian house I am still interested in an ASHP heater.

Is it optimal? Maybe not from a $$ perspective. But burning £300 of gas per month in the depths of winter does not seem a good thing to me.

We are insulating several outside walls over time. This has made a BIG difference to how warm the adjusted rooms feel.

I will definitely be giving Octopus a ring soon. Maybe after I have upgraded my rads.

Octopus will upgrade your rads as needed as part of the install. They are changing all but two of mine in Feb.

Do you mind me asking the cost?

If you divide your previous years gas kWh usage by 2900 you come to the power of the ashp you need to heat your home.

Maybe I’m a bit dim, but is this the max power of the ashp, or the actual amount of electricity you will be using. The reason I ask is that we have a 16kW ashp, but the actual usage is nowhere near that – rarely more than 6. At some point I had Octopus come out and do a survey and they measured our house and sucked their teeth and declared we’d need a unit bigger than they supply.

Octopus are picking the low hanging fruit, and their CEO stating with new high temp units you won’t need to upgrade rads is on dangerous ground. Octopus also sell electricity …….. Building regs now require  low temp heating systems as well, so a new system on a condensing boiler will need similar sized rads

I can upgrade my own rads cheaply enough. Our old ones were tiny, mostly 600×700 or smaller, with two in the big living room. I reckon I can double the nominal heat output by going slightly wider and double skin.

Molgrips: Our quote is crazy good, just under £2000 for a Daikin, 4 radiator upgrades (3 layer) and a 180l water tank inc installation and all electricals.

We got the quote when the subsidy was £5000 and then when it went to £7500 and Daikin had a £250 discount added it tumbled in cost. Neighbour go a quote a couple of months ago and his is more like £4000, so I think we were lucky and got in before the rush. 

@supersessions9-2 I’ve just installed similar whilst adding to and renovating  a 1965 cavity walled house. So far in the part of the house that is built to current building regs the heat pump works fine, not seen it pull more than 6kw. The second part of the renovation is to do the original house which will need interior wall insulation and careful attention to draught proofing.

We have underfloor downstairs only. The upstairs rads don’t seem overly large to my eyes.

Our sunamp is not charged from the 10kw ASHP, just from electricity. I need to sort out being on a different tariff for the winter as in the summer the 6kw west facing solar takes care of the sunamp and the other water tank.

I’m not sure if the position of the ASHP makes a difference. Ours is on the north facing side of the house and never gets any real sunlight on it.

We had an engineer do a set of calculations to size the heat pump which gives me confidence that it’s not undersized as long as I follow the renovation plan.